In particular, the invention deals with aircraft for which acoustic comfort, that is to say a level of noise which is as low as possible, is desired in areas where people, namely passengers or members of the crew, are located during the flight. The invention relates more particularly to aircraft for which acoustic comfort which can easily be modified in various areas is desired without compromising the equipment and fittings of the aircraft.
In an aircraft, various sources of noise emission create background noise in the areas occupied by people, namely passenger cabin areas or a crew station, which may, depending on the case, be considered to be a source of discomfort or irritation or even, in extreme cases, a source of tiredness.
Of the noise most frequently identified, noise originating from the engines, external aerodynamic noise and internal noise, in particular linked to air conditioning and equipment ventilation systems, may be mentioned.
In order to reduce the level of noise in a given area, the most common solution consists in employing materials which have what are known as acoustic absorption properties on the walls of the area, that is to say materials which have the effect of reducing the intensity of sound waves by causing accelerated damping of said waves.
Such materials, for example glass wool in the form of a blanket, are generally used in aircraft in the walls of the cabins between the structural lining of the fuselage and the trim panels of the cabin.
In this type of arrangement, the blanket containing glass wool plays a double role of thermal insulator and acoustic absorber and the trim panels protect the blanket and improve the appearance of the cabin.
However, this solution only improves the acoustic comfort to a limited degree because certain frequencies, in particular low frequencies in the audible range, cannot be effectively absorbed without utilizing large quantities of absorbent materials, this being unacceptable on board an aircraft due to weight and volume restrictions which characterize the design of aircraft.
Another known method for reducing the level of noise in spaces in general and in a cabin in particular is known by the expression active noise control.
Active noise control consists in measuring the ambient noise and generating a canceling noise by controlling the amplitude and phase of the canceling noise.
In order to generate the canceling noise, the solution consists in arranging in the space in which the noise level is to be reduced, for example a cabin, loudspeakers which are supplied in phase and intensity in order to generate sound waves which reduce by interference the level of sound energy in chosen areas of the treated space. This solution which consists in adding loudspeakers proves to be disadvantageous because of the weight and volume of the loudspeakers and the constraints of installing them.
The space in which loudspeakers can be installed is very often limited and there are thus problems of mechanical interference with other equipment and with the trim panels.
In particular, when the device for reducing noise is installed in an aircraft cabin, with the trim panels being currently designed initially without providing for the installation of a device for active noise control, mounting loudspeakers very often requires the shape of the existing trim panels to be modified in order to install the loudspeakers of the device for active noise control.
Moreover, noise reduction systems generally utilize numerous loudspeakers which cause problems in terms of cabling and electrical connections for their controls and power supplies and which also require the modification or replacement of existing trim panels and the fitting of new supports for holding the new electrical cables of the system, which cables must also be arranged respecting the severe limitations with respect to existing electrical installations for reasons of safety and good operation of the electrical systems of the aircraft.
Subsequent installation of an active noise reduction system in an existing aircraft proves to be particularly disadvantageous and requires numerous modifications which are difficult to carry out.